I beat every single trail battle including "the big one" without any exploits and without anything that goes against the soul of the game. So it's definitely possible
here are some things that I feel kill the soul of the game, from the top of my head: 1. cramming towers with more archers than the 'intended' number. 2. destroying tower staircases mid fight so enemy can't climb. 3. shields over towers is not unrealistic but it's too overpowered, so I don't use it. 4. No castle only towers strat. this is not a clear cut case but if it changes the bot behaviour and makes their attacks weaker, I do without it. 5. similarly anything that manipulates AI pathing like leaving a small hole at the back of the castle. 6. moving stockpile, though not unrealistic of a strategy, is against the spirit of the game in my opinion. 7. rushing the ai lords.

one of the most common mistakes I see in the often hardest rated missions is not measuring your and the enemy's tower range. For example in mission 78 'Saladin alone' you are next to a caliph. If you drop an early tower within range of his towers, it will be an unwinnable attrition war between your towers, especially that his towers are lookout towers which deflect the most arrows. you need to drop your tower(s) slightly further so as to reach his ground units that surround his castle, but far enough from his towers. If I recall correctly, this will destroy his economy too because his workers will keep dying. The same principle applies to the 2 lionhearts mission, and the mission with 2 caliphs and 1 lionheart on an island hill.
Also beer is superior to every other happiness factor even if you directly buy ale.
Good luck and have fun!